A nuclear reactor, such as a boiling water reactor (BWR) for example, includes a pressure vessel containing a reactor core including a plurality of fuel rods submerged in water. The pressure vessel is disposed in a drywell enclosure, which in turn is disposed in a containment vessel in an exemplary design.
Each of the fuel rods includes a plurality of nuclear fuel pellets contained in a tubular enclosure or cladding, which fuel undergoes fission during normal operation of the reactor for heating the water in the pressure vessel to generate steam which is conventionally channeled to a steam turbine-generator for producing electrical power for a utility grid for example. The fuel rods have useful lifetimes of several years and are continually shuffled into different positions within the reactor core, and removed and replaced with fresh fuel rods as required for attaining uniform burning, or fissioning, of the fuel rods. One of the by-products of the fission process is iodine, which may take the form of both stable or non-volatile species; or volatile species which vaporize, both of which species may be radioactive or non-radioactive. Typical iodine species include cesium iodide (CsI), hydrogen iodide (HI), and elemental iodine (I.sub.2). Iodine may also be found in organic species such as methyl iodide (CH.sub.3 I), and anomalous species which are presently unknown.
The various iodine species may either be stable or non-volatile such as the cesium iodide compound, or volatile such as the elemental iodine, the hydrogen iodide, the methyl iodide, and the anomalous species. The iodine species may also be either radioactive or non-radioactive, with the total amount thereof in a typical reactor core sized for generating about 1,000 MWe (megawatt electric) being about 10-20 kg after the reactor has been operated for several years.
As long as the fuel rod cladding remains intact, the iodine species will remain therein and not be released. However, in a postulated accident such as a loss-of-coolant accident (LOCA), one or more of the fuel rods may be overheated, which can rupture the fuel rod cladding and allow the iodine therein to be released into the reactor coolant water. Although only a very small fraction of the relatively small total iodine in the reactor core may be expected to be released into the reactor water following the LOCA, the volatile radioactive iodine species will vaporize and be released from the water and escape with the steam into and through the pressure vessel and in turn into the drywell, and further in turn into the containment through any leakage sites contained therein. The radioactive iodine will, therefore, be spread wherever it is allowed to travel, which increases the radioactive contamination area in the reactor plant. It is, therefore, an object of the present invention to decrease or prevent the release of volatile radioactive iodine from the pressure vessel following an accident condition such as the LOCA